1) Field of the Invention
The present invention relates to an improvement in the cell characteristics of non-aqueous electrolyte secondary cells.
2) Description of the Related Art
Non-aqueous electrolyte secondary cells, for their high energy density, are widely used as power sources for mobile appliances. In recent years, there has been rapid enhancement of functionality of mobile appliances such as mobile phones and laptop computers, resulting in a need for cells of higher capacity.
In order to meet this demand, an attempt is being made to enhance the efficiency of the positive electrode active material by charging the positive electrode to higher potentials (e.g., approximately 4.3 V on the basis of lithium).
However, enhancing the potential of the positive electrode causes generation of a low oxidative substance on the negative electrode. The substance moves to and reacts with the positive electrode, which poses the problem of degraded cycle characteristic and degraded resistance against continuous charging.
Examples of the prior art techniques related to non-aqueous electrolyte secondary cells include Japanese Patent Application Publication Nos. 2005-190996 (patent document 1), 2005-44675 (patent document 2), 2006-185793 (patent document 3), 2006-318839 (patent document 4), 4-308654 (patent document 5), and 5-159766 (patent document 6).
Patent document 1 discloses attaching lithium phosphate to the surface of a lithium nickel oxide. This technique is claimed to provide a cell that prevents decomposition of the electrolytic solution.
This document, however, gives no consideration to charging the positive electrode to high potential.
Patent document 2 discloses a separator of a lamination structure composed of a porous synthetic resin film sandwiched between felt separators mainly made of glass fiber.
This technique is claimed to provide a lead storage battery that prevents sulfation on the negative electrode plate and thus maintain its low-temperature rapid discharge characteristic for a long period of time.
This technique, however, is drawn to lead storage batteries and thus cannot be applied as it is to non-aqueous electrolyte secondary cells.
Patent document 3 discloses a separator with an air permeability of 60 to 400 sec/100 ml and with a porosity of less than 60%. This technique is claimed to provide a cell that excels in charge/discharge characteristic and usable under a high voltage of 4.4 to 4.6 V.
This technique, however, still cannot prevent the low oxidative substance, which is generated on the negative electrode, from moving to and reacting with the positive electrode.
Patent document 4 discloses addition of, to the non-aqueous electrolyte, cyclohexyl benzene, biphenyl, fluorobenzene, and t-alkyl benzene. This technique is claimed to provide a cell that is excellent in resistance against overcharge and does not swell after charged and preserved.
This document, however, gives no consideration to charging the positive electrode to high potential.
Patent document 5 discloses a separator made of a porous film with an aperture ratio of 50% or less and an aperture diameter of 0.3 μm or less. This technique is claimed to provide a cell that prevents internal short circuiting caused by dendrites.
This document, however, gives no consideration to charging the positive electrode to high potential.
Patent document 6 discloses a separator made of a porous polyethylene film with a film thickness of 20 to 30 μm, an air permeability of 200 to 1000 sec/100 ml air (ASTMD), and an average pore diameter of 0.02 to 0.05 μm. This technique is claimed to provide a cell that is excellent in safety against internal short circuiting and in current characteristic.
This document, however, gives no consideration to charging the positive electrode to high potential.